It also means that they literally want to stick something long and possibly attached to themselves up your butt hole.
The use of it essentially means either the user is:
-a homosexual male
-a red neck
-a horny homosexual male
-a female with the desire to test out her new strap-on
person2: "well.... up yurs"
person1: *blocks butt hole and runs away*
person1: "DON'T EVER GET NEAR ME AGAIN!